Preparation and Characterization of Molybdenum Trioxide from Spent Hydrodesulfurization Catalyst

Korean J. Chem. Eng., 28(7), 1546-1549 (2011) DOI: 10.1007/s11814-011-0005-9 INVITED REVIEW PAPER Preparation and characterization of molybdenum trioxide from spent hydrodesulfurization catalyst Barsha Dash†, Indra Narayan Bhattacharya, Bhaskara Venkata Ramanamurthy, and Raja Kishore Paramguru Institute of Minerals and Materials Technology, Bhubaneswar-751013, Orissa, India (Received 30 March 2010 • accepted 8 January 2011) Abstract−An approach to produce molybdenum trioxide from spent hydrodesulfurization (HDS) catalyst, obtained from a petroleum refinery, is presented here. The spent catalyst was devolatilized at 600 oC so as to make it free from oils, organics and other volatile species. It was then roasted with sodium carbonate at a temperature of 850 oC for 30 min. The leaching efficiency for 20% soda roasted sample at 10% pulp density was 99.8%. From the solution molybdenum was precipitated out as ammonium molybdate at pH 1.0 with HCl and ammonium chloride. This ammonium molybdate o was calcined at 750 C to get MoO3. The product was characterized by XRD. Its purity was determined titrimetri- cally and by ICP-AES. Key words: Spent Catalyst, Soda Roasting, Leaching, Molybdenum, Molybdenum Trioxide INTRODUCTION Table 1. Chemical composition of roasted catalyst Element Mo V Fe Cu Ni Co Zn Molybdenum is a metal of group six. It readily forms hard, stable Percentage 14.4 0.635 0.28 0.1 4 Nil 0.03 carbides, and for this reason it is often used in high-strength steel alloys. Silvery in appearance, molybdenum helps in keeping our environment green by desulfurizing carbon-based fossil fuels. Indus- EXPERIMENTAL trially, molybdenum compounds are used in high-pressure and temperature resistant greases as pigments and catalysts. As the demands 1. Materials for catalysts are increasing in modern industry, the utilization of The original hydrodesulfurization catalyst was crushed, sieved spent catalysts calls for research and developmental efforts. Many and roasted at 600 oC to remove oils and organics. The weight loss types of catalysts are used; however, the catalysts used in petro- was found to be 44%. The chemical composition of the devolatil- leum refineries have received much attention in recent years. Recov- sed catalyst is given in Table 1. ery of transition metals from spent petroleum refining catalysts is 2. Characterization an important problem. These industries mostly use molybdenum 2-1. XRD containing catalyst for desulfurization and mild hydrogenation pro- Phase determination by X-ray diffractometry study was done by cess. During the hydrorefining the catalyst gets poisoned due to con- using Phillips powder diffractometer PW 1830 X’pert system in tamination with different elements such as S, Fe, Si and Zn etc. the 2θ range of 10 to 70o with voltage of 30 kV and CuKα was used The only way to avoid disposing these spent catalysts directly in to characterize the samples. the environment is to process them, meeting the acceptable envi- 2-2. ICP-AES ronmental demands, to get a value added product. Various options The constitutional elemental analysis was done by ICP-AES of such as soda roasting [1] and salt roasting [2] are there for molybde- model No. Perkin- Elmer Plasma 4000 ICP-AES. num recovery where a water soluble molybdate product is obtained. 2-3. AAS Carbothermic reduction can also be done for hydrorefining spent cata- Other trace metals were estimated by Perkin Elmer 300A atomic lyst to extract molybdenum metal [3]. Molybdenum can also be re- absorption spectrophotometer. covered through hydrometallurgical processing via leaching with so- 2-4. Analyses dium carbonate mixed with hydrogen peroxide [4]. Direct ammonical Volumetric estimation of molybdenum was done by following or alkali leaching is another option for spent catalyst processing [5,6]. the complexometric determination of Mo [13]. EDTA reacts with Processing via fusion is also an option for utilization of spent catalyst MoV ion to form a stable complex compound. MoVI is reduced to [7]. Recovery of molybdenum compound from the leached liquor MoV by hydrazine in sulphuric acid medium in the presence of excess is another course like carbon adsorption-desorption studies [3,8] and EDTA which is titrated with a standard solution of zinc salt with solvent extraction [9-12]. In this paper the preparation and character- erichrome black T. The method is applicable to analysis of alloys ization of molybdenum trioxide from the HDS spent catalyst through containing Bi, Cd, Co, Zn, Ni, Cu, Hg, V, Cr and Pb. All accompa- roasting, leaching and precipitation has been carried out. nying elements are first titrated in one portion of the solution without reducing MoVI. The total amount of Mo and the accompanying ele- †To whom correspondence should be addressed. ments is titrated in the other portion after reduction. Substracting E-mail: [email protected] the former titration value from the later gives the value of molyb- 1546 Preparation and characterization of molybdenum trioxide from spent HDS catalyst 1547 denum titration. Volumetric estimation of vanadium was carried out by titrimetric determination method [14] of vanadium in ores with Mohr’s salt. The method is based on the titration of the pentavalent vanadium ion in a medium of 5N solution of sulphuric acid using standard solution of Mohr’s salt in the presence of excess phosphoric acid, which binds the Fe3+ ion in a colorless complex. 3. Roasting The catalyst was roasted in a 6”x6” electrically heated 4 kW muf- fle furnace (horizontal type). A charge consisting of devolatized HDS catalyst mixed with 20% (w/w) anhydrous sodium carbonate (Merck, India) was introduced in the furnace and heated to the desired temperature. The rate of heating was set as 5 oC per minute. The effect of temperature of roasting was studied from 550 oC to 950 oC while that of duration was carried out at 850 oC. Fig. 2. TG and DTA analysis of mixture of catalyst and sodium car- 4. Leaching bonate. Leaching of the soda roasted materials was carried out by taking 20 g soda roasted material with 100 ml water in a 250 ml conical flask fitted in a condenser. Heating and stirring were done through a hot plate with magnetic stirrer at around 250 rpm. After the leaching was completed, the slurry was filtered. The liquor contains dis- solved molybdenum and vanadium. 5. Precipitaion For the precipitation of the leach liquor, ammonium chloride (Merck, India) and HCl (Merck, India) were added in two sequences, i.e., route 1 and route 2. In route 1 HCl was first added to acidify 40 ml leach liquor to pH 1.0 taken in a 100 ml glass beaker. After that 8 g ammonium chloride was added with stirring by a magnetic stirrer. In another two similar sets of experiments, 1 g and 3 g ammonium chloride were aded to study the variation in the amount of Fig. 3. Variation of roasting temperature, Condn: 20% soda, time- ammonium chloride. In route-2 again 1, 3 and 8 g of ammonium 1h. chloride were added followed by addition of HCl upto pH 1.0 in each case. Precipitates obtained from all cases through both the routes and other volatile matters and organics. TG and DTA analysis of were filtered and dried in a hot air oven at 80 oC for 24 h. soda (20% w/w) mixed catalyst was done, which is shown in Fig. 2. The first weight loss of 7.84% was observed at 100 oC, showing RESULTS AND DISCUSSION the loss of moisture. Then the second weight loss for the rest of the region up to 1,000 oC was 5.34% in the thermogram. This loss can The detailed chemical analyses, spectrophotometric analyses, be attributed to loss of carbon dioxide. leaching procedure and precipitation are given below in different sec- In the process of soda roasting, molybdenum trioxide reacts with tions. Fig. 1 shows the XRD pattern of the devolatilized catalyst. It in- sodium carbonate giving sodium molybdate and carbon dioxide. dicates major presence of alumina, MoO3 and NiO to a lesser extent. The reaction is as follows: 1. Roasting MoO +Na CO →Na MoO +CO (1) The catalyst was initially devolatilized at 600 oC to remove oils 3 2 3 2 4 2 The roasting temperature was varied from 550 to 950 oC and the percentage of extraction of Mo in the leach liquor was plotted as shown in Fig. 3. The extraction of molybdenum was in increasing order up to 850 oC. The highest extraction was found at 850 oC and above due to the complete diffusion of sodium carbonate. This is because the melt- ing point of anhydrous sodium carbonate is 850 oC. At 750 oC and above molybdenum has a tendency to form polymeric species. But 2− when it is alkalized the polymolybdate ion converts into MoO4 [15]. 2− Thus, molybdenum forms soluble sodium molybdate (Na2MoO4 ). The kinetics of roasting is given in Fig. 4. The time period of 45 minutes was optimized. 2. Leaching Fig. 1. XRD of devolatilized catalyst. Leaching experiments were carried out by taking the soda roasted Korean J. Chem. Eng.(Vol. 28, No. 7) 1548 B. Dash et al. Table 2. Recovery of Mo from route 1 Amount of NH4Cl Recovery of Mo 8g 94.24% 3g 94.19% 1g 90.03% Table 3. Recovery of Mo from route 2 Amount of NH4Cl Recovery of MoO3 8g 89%00. Fig. 4. Kinetic of leaching with soda roasting of spent catalyst, 3g 87.5%0 Condn; temp.- 850 oC, 20% sodium carbonate. 1g 78.44% (28 oC) to 80 oC as shown in Fig. 6. At 28 oC the leaching efficiency was found to be 77%, and it increased to 99% at 80 oC.

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